This invention relates to an oil pressure control apparatus, and in particular an oil pressure control apparatus used for a valve timing control device that controls a timing of opening and closing of an intake or an exhaust valve of an internal combustion engine in accordance with engine operating conditions. Many different types of hydraulic actuator, as a operating valve timing control device, have been proposed. One such hydraulic actuator includes a source of hydraulic pressure and a control valve that is disposed between the source of the hydraulic pressure and the hydraulic actuator for controlling the hydraulic pressure introduced into the hydraulic actuator from the source of the hydraulic pressure. The control valve comprises a valve body, having a plurality of ports that are opened on external surface thereof, that is connected to the hydraulic actuator and the source of the hydraulic pressure. The control valve also includes a valve spool, which is slidably received in an internal chamber of the valve body for opening and closing the ports, and is operated by a plunger that is actuated by an electromagnetic coil. A conventional device embodying this kind of the oil pressure control apparatus is disclosed, for example, in Japanese unexamined publication (koukai) 6-330712. The hydraulic actuator also comprises a filter that is disposed between the source of the hydraulic pressure and the control valve so as to prevent foreign matter from being introduced into the control valve in order to avoid accidental operation of the control valve. As an example, U.S. Pat. No. 5,797,361, such a filter is only disposed between the source of the hydraulic pressure and the control valve. Therefore, this conventional device is capable of filtering the oil from the source of the hydraulic pressure, but it is not capable of filtering the oil circulating through the hydraulic actuator. In this case, if foreign matter is present in the hydraulic actuator, it would be trapped in the oil circulating through the hydraulic actuator and might be introduced into the control valve. In addition, in this case, the foreign matter flowing together with the oil might cause the accidental operation of the control valve.